In a floating gate electrode type nonvolatile semiconductor memory device, as an inter-electrode insulating film (IPD: Inter-Poly Dielectric) that insulates a Floating Gate (FG) and a Control Gate (CG) from each other, a laminated insulating film of a silicon oxide film/silicon nitride film/silicon oxide film (an ONO film) has been conventionally used. The silicon nitride film (an SiN film) in the ONO film does not have a perfect stoichiometric composition but a slightly silicon-excess composition because of its forming method. In the silicon-excess SiN film, a level for trapping electric charges is present, and hence some of electrons are trapped by the level in the SiN film even if the electrons leak at the time of application of an electric field (e.g., a write/erase operation). Further, a barrier height of the silicon nitride film is lower than a barrier height of the silicon oxide film, both the barrier of a track level in the SiN film and the barrier height of SiO2 films formed to sandwich the SiN film serve as a barrier in the ONO laminated structure, whereby trapped electrons are hardly detrapped (leak) to the outside of the ONO insulating film.
An electric field applied to the inter-electrode insulating film must be reduced to be lower than that of a tunnel insulating film, leak to a control electrode side at the time of a write operation must be suppressed, injection (storage) of electros to a floating gate must be activated, and hence the inter-electrode insulating film is formed into a three-dimensional structure so that a high capacity can be assured even though a film thickness is large. As a result of taking the three-dimensional structure, a step is formed between the floating gate and an element isolation insulating film, and each corner portion (an edge portion) is present at a top portion of the floating gate and a bottom portion between cells. Since an electric field is concentrated on the edge portion, a leak current in the inter-electrode insulating film flows through the edge portion on a preferential basis at the time of a write/erase operation. As a result, electron trap into the inter-electrode insulating film more excessively occurs at the edge portion, and a concentration difference is produced in an electron trap amount in the continuously formed inter-electrode insulating film.
Although trapped electrons in the inter-electrode insulating film hardly leak to the outside of the ONO insulating film, redistribution in a transverse direction (a direction orthogonal to a thickness direction) occurs so that the concentration difference is eliminated in the inter-electrode insulating film. When the redistribution of the trapped electrons to the inter-electrode insulating film occurs while electric charges are held after end of the write/erase operation, a threshold value fluctuates, and charge retention characteristics are deteriorated.